There is no admission that the background art disclosed in this section legally constitutes prior art.
Carbon nanotubes (CNTs) have attracted world wide attention because of their distinctive mechanical, chemical and electronic properties. Recently, CNT-based gas sensors [2-10] have received considerable attention because of their outstanding properties such as a faster response, higher sensitivity, and lower operating temperature. CNT gas sensor successfully detected ppm level H2, NO2, CO2, O2 and NH3.
In a slightly different mechanism, dielectric phoresis impedance analysis 3, F2, NH2, and NO2 has been detected. These CNT based sensors have advantages over metal oxides due to detection capability in room temperature. The CNTs have attracted great interest for use as gas sensors mainly due to their large surface area, high electrical conductivity and chemical stability while the surface reacts with gas molecules [10-14]. CNT-based sensors have been investigated for the detection of gases of [15-20] as well as of other chemicals like organic compounds [21,22]. Both single-wall carbon nanotubes (SWNTs) and multi-wall carbon nanotubes (MWNTs) have been used as the sensing probes. For device fabrication, either CNTs were directly synthesized on substrates via chemical vapor deposition (CVD), or the collected CNT powders were pasted on substrates. In some cases, carbon nanotubes have been modified by coating a catalytic metal particles, metal oxides and organic materials to enhanced selectivity of particular gas. For instance, SnO2 were coated by vacuum evaporation method to detect Co selectively, while Pd clusters were coated on nanotube surfaces by a wet chemical method to enhance hydrogen detection capability.
Chlorine gas is widely used as effective whiteners and disinfectors in many industrial processes such as paper, fabric, water purification and food production [2-5]. Chlorine is very harmful when emitted into the environment. In small amounts, chlorine and chorine dioxide exist in the atmosphere and are ozone-decomposing impurities [1]. Moreover, chlorine is one of the constituent elements of toxic dioxin. Chlorine containing compounds have been used as chemical weapons throughout history. Thus, the problem of the rapid determination of chlorine and chlorine dioxide in the air at the level of their maximum permissible concentrations, 1 and 0.1 mg/m, respectively, is important. Though highly useful, detection of a trace amount of chlorine is very important to avoid environmental destruction and safety purposes. Also, in order to monitor emission of chlorine gases to the environment, continuous detection capability is important.
Several solid state electrochemical sensors have been developed for the measurement of low chlorine concentrations. These sensors operate at relatively high temperatures and use metal chlorides and AgCI/Ag-β″-alumina as the solid electrolyte. The emf obtained with such devices in chlorine-free environments remains high, and Nerstian responses are impossible to obtain for partial pressures less than 10.5 atm.
The design and testing of a chlorine gas potentiometric gauge, using SrCI/KCI as the electrolyte have also been reported. Though this sensor can detect lower concentrations in 10 ppm range in conventional diluting gases such as air, argon, oxygen and nitrogen, the sensor operates between 120 and 400° C. which cause some difficulty in continuous monitoring.
Also, marketed versions of instruments for chlorine measurement already exist and are very effective for high concentration of chlorine, in the range of 1-10.4 atm. These spectrophotometric UV devices operate in continuous mode at room temperature and with excellent reliability. Their high costs may, however, prevent their use in certain installations.
There are a few approaches to develop chlorine sensors using metal oxide semiconductors. Among these metal oxides, commonly used materials are SnO2, ZnO and WO3 have been tested in practical testing devices. It is believed that a resistance decrease in semiconductor gas sensors exposed to inflammable gases results from desorption of oxygen adsorbed on the surface and grain boundaries of metal oxides at high temperatures in the air. However, this sensing mechanism is not expected in the detection of chlorine gas. In addition, Chlorine gas may corrode metal oxides at high temperatures in air because of the HCl resulting from the chemical reaction with the water in air.
Therefore, for the practical use of chlorine semiconductor gas sensors, it is necessary to develop sensor materials with high sensitivity as well as high stability.
In order to stabilize metal oxides in chlorine ambient, multicomponent oxides-based gas sensors were developed. However, these sensors could detect chlorine only above 250° C.
In resistive sensors, phthalocyanine films have been investigated. Though these sensors have high sensitivity down to ppm level, the stability of these polymer films were very poor under elevated temperatures compared with metal oxides.
Other detection methodologies include gas chromatography and chemical analytical methods which requires time, expertise and costly instrumentations. Also, these detection methods cannot be used for continuous monitoring of ambient at multiple places rapidly.
As discussed above, there is a growing demand for efficient detection methods of chlorine gas. It is desired that this method be cost effective, easy to operate and reliable. Though UV spectroscopy based detection method has been commercialized, the use of such devices in outdoor applications, environmental monitoring and online detection is difficult.
Another method used metal oxide based-sensors in order to attempt to provide a low cost and simple detection of hazardous level of chlorine leakages. The disadvantage of such detection method is the heating requirement that reduces the lifetime of the sensors at elevated temperatures. Due to the same heating requirements, operational costs of these sensors continue to make monitoring difficult. Another difficulty is that change of batteries makes inconvenient to use metal oxide sensors.
Therefore, development of gas sensors for rapid monitoring of ambient environments using an inexpensive and straightforward method is required.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.